Rick Denney wrote:
Ed writes...
You need to measure this for forward/reverse resistance, not continuity.
This diode acts as a fuse to protect the metering circuit. With no drive
applied, you should see no grid current.
Bingo! D16 was a dead short. I replaced it, and the amp now seems to
work fine.
Good deal.....Good job...keep in mind this also is in the B- return line
and it needs to "float" above ground to complete the cicuit.
Another thing that bothers me is the ALC circuit. again an old and
proven design. However it picks off some of the RF output, rectifies
it and sends it back to the input circuit. If it were me I would
take this out of the RF path, especially since TT does not provide
ALC.
If I end up with a Centaur, this will be my second operating position
when I install a new antenna later this year, and that radio (a
Kenwood) has ALC.
Still not needed. ALC was used years ago when most transmitters were
capable of running in excess of 100 watts.
Keep in mind that TT stopped producing the Centaur, not because it
didn't sell, but they could not find acceptable tubes.
What is meant by "acceptable"? Does this mean that the Centaur has
different requirements than the Ameritron in terms of tubes? I figured
I could just order the Chinese-made Taylor Select tubes from RF Parts,
and not drive them as hard.
No the requirements are the same. The Chinese tubes are too un-reliable
and finding 2 with the same operating characteristics are next to
impossible to find. The ideal situation would be 3 matched tubes. If it
were me, I would look for NOS Jan tubes. And forget the 572's, there is
no advantage other than a minimal increase in output.
I'm not going to comment on the parasitic discussion other than to say
that most catastrophic events in an amp are caused by component failure
and/or operator error.
When tuning on 10 meters, I overdrove the grid and plate currents. I
was looking at the wrong meter (the wattmeter) and overdrove the amp
during tuning. Also, for a while now I've been tuning the amp using
the Tune control, and switching the amp with the Relay out from the
Omni V. This combination apparently hot-switches the amp. I suspect it
was the overdriving that caused the problem. I wonder if the old
suppressor design's inductor wasn't bypassing all that excessive 28
MHz RF around the resistor, causing it to overheat and blow. For all I
know it may have already been blown, waiting for me to use it on 10
(which I had not done before).
Anything I should look at carefully before declaring victory?
**** KEEP IN MIND HV CAN AND WILL KILL YOU **** BE CAREFUL ****
Oh, yes.
Thanks for your excellent help.
Rick, KR9D
I would recommend tuning the amp with some type of pulser arrangement.
The best method is to run your keyer up to 40+ wpm and tune the amp in
the CW position. Or you could build/buy a pulser unit for SSB. Do not
tune the amp at full out from the rig, 50-60 watts drive is plenty on 811s'.
There are several circuits around that will prevent the hot switching. I
think Harbach sold one at one time as well as Rick Measures, but I'm not
real sure on that. Also check the TT archieves as I know this has been
discussed before.
Last piece of advice before I go eat breakfast. If you plan on repairing
your amps, get an ARRL or Bill Orr handbook and study up on the theory
of how an amp works. Take a look at some of the circuits and what they
do and how they operate. A dead amp with no visible signs of damage is a
difficult animal to fix.
Glad I was able to help....I enjoy giving back to Ham Radio for what it
has given me for the last 30 years........73
Ed W3NR
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